In the mining industry many minerals such as alluvial gold, lead, zinc, tin and barite are often beneficiated at least in part by density separation systems. Present techniques such as heavy media separators, jigs, or tables can be employed to accomplish these tasks, however the cost of performing such separations on a large scale can be quite high. Other more dynamic methods such as sluice boxes must be operated on a batch process basis, and are often inefficient with respect to recovery of particles at the fine end of the particle size spectrum.
Forms of elutriation techniques have been in use in laboratories for decades where particles are classified according to diameter, as in a "Cyclo-sizer" (Trademark of Warman International). This system utilizes the differential sedimentation rates of particles with different diameters to effect separations according to size. In this application particle density must be constant in order for consistent size classifications to be made.
A patent of background interest to the present invention is U.S. Pat. No. 2,429,436, issued Oct. 21, 1947 to G. B. Walker. Walker teaches that the "dynamic sedimentation rate" of a particle, that is the rate at which a particle under the influence of gravity settles against a vertical flow of "separation medium", is determined by the partical diameter and density, and by the density and flow rate of the separation medium. (The separation medium is described as a liquid with solids in suspension.) Thus, where particles are settling at an equal rate against a given flow, the smaller particles of this group will be of relatively higher density with respect to the larger particles settling at the same rate. Walker then outlines how a screening step following the dynamic sedimentation or flow step is employed to separate the large particles from the smaller higher density particles. Thus by applying this screening step to the underflow of a given vertical flow rate, a density separation affect can be achieved, where the undersize underflow will comprise the high density material, and the oversized underflow will be of relatively lower density.
Walker further teaches how this idea can be applied to effect density separation over a wider size spectrum by starting with a relatively high separation medium flow rate in the first separation chamber such that the "sink" or underflow material which settles out is essentially comprised of only the largest of the denser particulate constituents, and thus does not require screening. In Walker's preferred embodiment, the overflow, or the "float" fraction from the first separated flows freely into a second separator in which the upward flow of the separation medium is adjusted to permit the settling of somewhat finer dense particles and the largest of the lighter particles. The sink product of the second separator is now subjected to a screening process wherein the screen mesh size is selected to allow the smaller, more dense, particles to pass while the larger less dense particles are retained. Correspondingly the float from the second separator is introduced into a third which has an even lower vertical flow velocity of separation medium such that the finer fractions in the particle spectrum can be addressed. This trend continues through the system in order to separate finer and finer fractions at each step until the system is no longer economical to operate as the mass of the remainder decreases. However, as the discharge of float from each step is approximately equal in volume to the sum of the inflow plus the vertically flowing separation medium, each step in the series must be physically larger than the previous step in order to accommodate the increasing inflow. This problem effects the economic utility of the system, and becomes especially critical when multi-separator systems are required to perform density separations over a wide size spectrum.
Walker further teaches how the density of the separation medium can be increased by suspending high density colloidal or semi-colloidal solids in the liquid being employed. By increasing the density of the separation medium towards that of the less dense material, the ratio of the diameter of low and high density particles which will settle at the same rate through such a liquid increases. This allows for the flow or elutriation based operations to be affected by partial density to a greater extent. On the other hand, both the high and low density particles will settle at a somewhat lower rate, thus the system cannot be operated at quite as high a rate.
Walker specifically describes how a high density magnetizable powder can be carefully diluted to the desired density, used as the separation medium, reclaimed by a magnetic separator, demagnetized by passing it through coils in order to prevent magnetic agglomeration, then thickened prior to redilution for reuse in the separation medium. Unfortunately, the process for demagnetizing such a material, or thickening a colloidal or semi-colloidal suspension both pose major technical problems. Further, a magnetic separation medium could cause problems during screening operations by sticking to itself, to other particles or to the screens. Walker stated that if such a magnetizable material cannot be found or employed for some other reason the same liquid weighting principles can be used, however a gravity separation system will be required to recover the weighting agent. This again presents a formidable technical problem, this being performance of gravity separation on particles which are colloidal or semi-colloidal in size. Thus utilizing a medium of this nature in order to perform a density separation simply results in having to perform another more difficult density separation.
In practice, the shortcomings of this design become more evident. Many have already been pointed out regarding the shortcomings of the process, as described by Walker, but the apparatus itself, as described by Walker, is also fraught with inherent inefficiencies, and logistical problems which will now be outlined. In his preferred embodiment, Walker illustrates and describes the series of separators as inverted cones. The angle of the apex of these cones is apparently in the range of 45 to 90 degrees, and the cones are equipped with a spigot having an outlet valve, and a "bustle" for the admission of the separation medium. Firstly, the effective area through which the separation medium flows at the desired speed or "design velocity" is at the apex of the cone, and is quite small. This area represents the effective area across which the separation is made, thus limiting it will limt the rate at which the separation can be made. Further, as the area across which the separation is being performed is so small, the design velocity will accordingly increase as the effective separation area is decreased by underflow particles passing through the area. Another problem relating to maintaining the design velocity across the effective area is that Walker includes no provision to ensure "plug flow" is achieved in the effective area. That is to say that the actual upward velocity of the separation medium will vary across the effective area of the separator. These variations in flow velocity will inevitably lead to variations in the sedimentation characteristics of the underflow and result in errors in the density classification. Another shortcoming of note is that Walker makes no provision to prevent the separation medium from simply flowing through the discharge spigot without effecting the required vertical flow through, the upper portions of the one cone as required. Therefore a considerable portion of the separation medium is not performing its designated task. The remainder of the separation medium which does flow upward quickly loses speed as the cross sectional area of the cone rapidly increases. This geometry results in a large fraction of the material to be separated remaining trapped in the upper portion of the cone since it is too small or of insufficient density to escape as underflow, but too large or dense to be floated off as overflow. Given time this situation will result in bedding of the material in the cone with channels running through the beds to allow the separation medium and the inflow to flow directly into the next cone. The obvious solution to this problem is to reduce the size of the cone such that the overflow takeoff occurs before the velocity is reduced to any appreciable extent. However, due to the variations in the design flow profile, as outlined above, early overflow of the float product would result in some of the particles, which should have been underflow, being erroneously contained in the overflow.
It is the object of this invention to effect density separations by means of combining screening operations with flow related separation steps in such a ways as to overcome the problems inherent in the apparatus such as that of Walker.